The actions of thyroid hormone (thyroxine (T4) and triiodo-thyronine (T3) on adenosine triphosphatase (ATPase) activities are to be defined in the intact rat renal tubule cell and in human erythrocyte (RBC) membrane systems. The already demonstrated effect of iodothyronines on Ca2 ion-ATPase in the RBC is to be examined from the standpoint of hormonal facilitation of calmodulin interaction with the enzyme. The Na,K-dependent ATPase system ("sodium pump") is to be studied in concert with the Mg2 ion-ATPase, since changes in either ATPase will influence calculation of Na,K-ATPase activity inferred from conventional ouabain-susceptibility techniques. Time course of action, hormone dose/response curves and hormone analog studies are to be conducted on the ATPase in both model systems. The time course and amount of RBC membrane uptake of T4 and T3 is to be compared with the time courses of iodothyronine stimulation of ATPase activities.